Research On Thermal Management And Electrochemical-Thermal Coupled Characteristic Of Lithium-ion Batteries

Lithium-ion battery is widely used in electric vehicles(EVs)because of their high energy density and low self-discharge rate.However,the high heat generation rate during the charge and discharge process of lithium-ion batteries,especially under high discharge rates or high temperature conditions,can easily cause a large amount of heat accumulation and overheating of the battery,which will seriously affect the performance and life span of the battery,and may lead to thermal runaway of the battery and cause serious safety accidents eventually.In this theses,from the perspective of thermal safety of cylindrical lithium-ion batteries,the research on battery thermal management systems(BTMS)was carried out,including the improvement of battery cooling performance and electrochemical thermal coupling characteristics.Firstly,for the cylindrical 18650 lithium-ion battery,the heat generation model of single battery and the heat dissipation model of battery pack were established,and the air-cooling heat dissipation was studied.The temperature rises of single lithium-ion battery under different discharge rates,different radial thermal conductivity,different convective heat transfer coefficients and different ambient temperatures was studied;The effects of different arrangement structure,different external environment characteristic parameters and different thermophysical parameters of the battery on the heat transfer and distribution of the battery were analyzed.It is found that higher discharge rate and ambient temperature,lower radial thermal conductivity,convective heat transfer coefficient and higher ambient temperature will aggravate the non-uniformity of temperature difference between single cells;The larger the distance between batteries,the lower the maximum temperature and the smaller the temperature difference,the better the heat dissipation effect;Considering the energy density and cooling effect of the battery pack,the hexagonal structure is the best;The more anisotropic the battery thermal conductivity is,the less uniform the battery temperature distribution is.With the increase of the battery radial thermal conductivity,the more uniform the battery pack temperature distribution is.According to the structure of cylindrical lithium-ion battery,the internal cooling system of PCM battery was constructed by replacing the internal hollow shaft with the phase change material(PCM)mandrel.The performance of the internal and external cooling systems of PCM and the relationship between the size of PCM mandrel and the overall energy density of the battery system were compared.It is found that PCM internal cooling can achieve the dual goals of reducing the battery internal temperature and improving the uniformity of temperature distribution;increasing the size of PCM mandrel can improve the cooling effect and reduce the overall energy density of the battery;compared with PCM external cooling,PCM internal cooling does not change the appearance of cylindrical battery,so it is more suitable to be used as an auxiliary cooling method in BTMS.According to the internal electrochemical characteristics of the battery,a one-dimensional electrochemical thermal coupling model was established with a single-layer cell as the research object.Based on charge conservation,mass conservation,electrode reaction kinetics and energy equation,it give a description of the electrochemical reaction process and heat transfer process inside the battery.Under different operating temperatures,the model was used to calculate the concentration of Li~+,local current density and the change of overpotential.Besides,the effect of temperature on the electrochemical characteristics was analyzed from the perspective of micro mass transfer.